In a few versions add-ons won't work anymore because "they are too powerful" (/"have more potential for malicious content"). They are going to be replaced with WebExtensions which aren't as powerful.
Afaik you won't be able to change the look of Firefox for example. I'm currently using classic theme restorer to get rid of all the unnecessary search things they put into the url bar. In the latest versions they added for example the first entry shown here: ( https://cdn.ghacks.net/wp-content/uploads/2015/12/remove-visit-firefox-search.jpg ) and buttons at the end of that list to search for what you typed in the url bar with different search engines. But you can't switch both of those additions of neither in the options nor in about:config.